The present invention relates to pulverized coal-fired furnaces and, more particularly, to a method and apparatus for deflecting as it discharges from coal-air and auxiliary air nozzles into the furnace enclosure.
One method of firing coal in conventional coal-fired steam generator boilers is known as tangential firing. In this method, pulverized coal is introduced to the furnace in a primary air stream through nozzles, termed coal-air admission assemblies, disposed in windboxes located in corners of the furnace. Each windbox comprises a vertical array of alternate auxiliary air compartments and coal-air admission assemblies. The coal-air and auxiliary air streams discharged from these burners are aimed tangentially to an imaginary circle in the middle of the furnace to create a fireball therein extending over the height of the windbox.
A distinct advantage of the tangential firing concept is that a wide range control of steam temperature can be obtained by physically raising or lowering the fireball within the furnace. By physically raising the fireball within the furnace, the heat absorption by the furnace bounding walls, and therefore the heat lost by the combustion gases, is decreased such that the temperature of the combustion gases leaving the combustion zone and passing over the superheat and reheat convection surface located downstream of the furnace is increased thereby increasing the potential outlet steam temperature. Similarly, by physically lowering the fireball within the furnace, the heat absorption by the furnace bounding walls, and therefore the heat lost by the combustion gases, is increased such that the temperature of the combustion gases leaving the furnace and passing over the superheat and reheat convective surface located downstream of the furnace is decreased thereby decreasing the potential outlet steam temperature.
In the prior art, it is common to physically raise and lower the fireball within the furnace by tilting the nozzle tips of the coal-air admission assemblies and the auxiliary air compartments disposed in the corner windboxes upwardly or downwardly in response to steam temperature measurements see for example U.S. Pat. Nos. 2,363,875 and 2,575,885. Presently, the nozzles of the coal-air admission assemblies and the auxiliary air compartments are linked together by a mechanical linkage so as to tilt in unison either automatically in response to steam temperature by means of an actuator or by manual adjustment. However, the linkage mechanisms for accomplishing such tilting requires many moving parts.
More recently, as disclosed in U.S. Pat. No. 4,294,178, it has been found desirable to admit the coal-air streams and auxiliary air streams into the furnace at different firing angles for purposes of controlling the formation of nitrogen oxides, a noxious pollutant, and to control slagging of the bounding furnace walls. In particular, it has been found desirable to aim the coal-air streams into the furnace towards an imaginary circle near the center of the furnace and to aim the auxiliary air into the furnace along the bounding walls so as to form a central fireball wherein the coal is burned under substoichiometric conditions surrounded by a sheet of air along the bounding furnace walls. In present operation, this is accomplished by directing the coal-air streams into the furnace at a set angle and directing the auxiliary air streams into the furnace at varying angles through a nozzle tip which may be tilted in the horizontal direction as well as in the vertical direction as described above for steam temperature control. Necessarily, the mechanical linkage for tilting the auxiliary air nozzle tips is more extensive in order than the nozzle tip may be adjusted in a horizontal plane and simultaneously in a vertical plane.
In addition, when a particularly heavy slagging coal is fired, it is possible for the slag deposits to bridge between the nozzle and the bounding wall so as to lock the nozzle tip in a set position. When this occurs and the operator attempts to adjust the nozzle tilt in order to raise or lower the fireball within the furnace, the linkage mechanism associated with the locked nozzle tip or the nozzle tip itself may be damaged.
It is an object of the present invention to provide a method and apparatus for deflecting a coal-air stream discharging from a coal-air nozzle and an auxiliary air stream discharging from an auxiliary air nozzle in a vertical and/or horizontal direction without resort to such mechanical linkages and the many moving parts associated therewith.